Gold is typically recovered from ores using a conventional cyanidation leach process. In the process, gold reacts with cyanide and oxygen by the following reaction:4Au+O2+8CN−+2H2O→4Au(CN)2−+4OH−  (1)
Gold is usually then recovered from solution using activated carbon as an adsorbent. Ion exchange resins may also be used to adsorb the gold cyanide complex, followed by elution with an acidic thiourea mixture. Thiosulfate leaching is a potential environmentally acceptable alternative to cyanidation and, in this process, the gold is leached as the gold thiosulfate complex. However, this complex is not readily adsorbed by activated carbon and so anion exchange resins may be preferred. Other metals, such as copper and mercury, also adsorb onto resins concurrently with gold.
The thiosulfate leach process has been demonstrated to be technically viable for a range of different ore types. For instance, Berezowsky et al., U.S. Pat. No. 4,070,182, disclose a process to leach gold from copper-bearing sulfidic material with ammonium thiosulfate. Kerley Jr., U.S. Pat. Nos. 4,269,622 and 4,369,061, disclose using an ammonium thiosulfate leach solution containing copper to leach gold and silver from ores containing manganese. Perez et al., U.S. Pat. No. 4,654,078, disclose leaching gold and silver with a copper-ammonium thiosulfate lixiviant to produce a pregnant leach solution, from which gold and silver are recovered by copper cementation. In these processes, ammonium thiosulfate is the preferred lixiviant, which results in the production of a tailings product which contains ammonia/ammonium ions. This is of concern from an environmental perspective. A leach process incorporating non-ammonium sources of thiosulfate, including sodium thiosulfate and calcium thiosulfate is therefore preferred.
Following leaching, gold may be loaded onto resins from either a slurry or a solution, and the gold is subsequently recovered from the resin by elution or desorption. Gold can be eluted from resins using eluants, such as thiocyanate, polythionate or nitrate based eluants. However, relatively concentrated solutions are required for the elution process. For example, in a nitrate elution process, 2M ammonium nitrate is preferred as disclosed in PCT Application No. WO 01/23626. This is a relatively high concentration of nitrate that creates demonstrable cost implications for the elution step and environmental impacts in disposing of spent ammonium nitrate solutions.
Thiocyanate solutions are known to rapidly elute gold (either cyanide or thiosulfate complexes) from resins. However, the resin must be regenerated prior to addition back into the resin-in-pulp circuit; otherwise, the thiocyanate will accumulate in process water, eventually leading to environmental problems and reduced gold loading. In addition, the loss of thiocyanate may be economically unacceptable. Regeneration in the thiocyanate system is also complicated as thiocyanate is removed using ferric sulfate followed by regeneration of thiocyanate by addition of sodium hydroxide. The rapid change in pH in the elution and regeneration steps produces osmotic shock in the resin and this leads to resin loss through breakage. A number of chemical reagents are also required at a plant site that may be remote. It is therefore desirable, subject to plant operational efficiency, to reduce the inventory of different chemicals used in plant operation. An aim is to use fewer reagents in lesser quantity.
A polythionate eluant system utilizes a mixture of trithionate and tetrathionate. Since these species are strongly adsorbed on a resin, they can be used to effectively elute gold. The high affinity of polythionates for the resin necessitates a regeneration step. Regeneration is accomplished by treating the resin with sulfide, bisulfide, or polysulfide ions to convert the polythionates to thiosulfate. A problem with polythionate elution is the stability of the tetrathionate solution. In the presence of thiosulfate, tetrathionate undergoes a decomposition reaction to form trithionate and elemental sulfur, and in the presence of silver or copper, decomposes to precipitate copper or silver sulfides. Trithionate decomposes to form sulfate, especially when present in high concentrations. Such decomposition reactions result in losses that add to the cost of the process.
In United States Patent Application 2011/0011216, it is shown that the addition of sulfite ions to various eluants enables the elution to be conducted with lower concentrations of reagents. A mixed trithionate/sulfite system is shown to be especially effective at eluting gold from the resin.
There is a need to provide a process for recovery of metals by ion exchange which gives high elution efficiency but does not generate waste solutions or resins, which contain undesirable species that either cause issues with their disposal or recycle back to the process.